Between 480,000-507,000 people were killed in Afghanistan, Pakistan and Iraq in the wake of 9/11 attacks, study says.

Hundreds of thousands of people in Afghanistan, Iraq and Pakistan have been killed due to the so-called "war on terror" launched by the United States in the wake of the September 11, 2001 attack, according to a new study.

The report, which was published on Saturday by the Brown University's Watson Institute for International and Public Affairs, put the death toll between 480,000 and 507,000.

The toll includes civilians, armed fighters, local police and security forces, as well as US and allied troops.

The report states that between 182,272 and 204,575 civilians have been killed in Iraq; 38,480 in Afghanistan; and 23,372 in Pakistan. Nearly 7,000 US troops were killed in Iraq and Afghanistan in the same period.

The paper, however, acknowledged that the number of people killed is an "undercount" due to limitations in reporting and "great uncertainty in any count of killing in war".

"We may never know the total direct death toll in these wars," wrote Nera Crawford, the author of the report titled "Human Cost of the Post-9/11 Wars: Lethality and the Need for Transparency".

"For example, tens of thousands of civilians may have died in retaking Mosul and other cities from ISIS [also known as ISIL] but their bodies have likely not been recovered."

'War remains intense'

People who were indirectly killed as a result of war, such as through disease or bad infrastructure, were also not included in the report.

In a statement, Brown University said the new toll "is a more than 110,000 increase over the last count, issued just two years ago in August 2016".

"Though the war on terror is often overlooked by the American public, press and lawmakers, the increased body count signals that, far from diminishing, this war remains intense."

As an example, the US war in Afghanistan, which has been the country's longest military invasion for 17 years, has lessened in intensity in recent years, but the number of civilians killed in 2018 has been one of the war's highest.